1. Field of the Invention
The present invention relates to a projector, and more particularly to a projector having a function to correct distortion of an image that is projected obliquely onto a screen.
2. Description of the Related Art
Projectors that employ image forming devices, such as liquid crystal light valves, digital micromirror devices, or the like, which are arranged to produce a light beam that is modulated based on an image signal and to project the modulated light beam onto a screen at an enlarged scale, have been widely used for the purposes of presentations and other image display occasions.
In actual applications of projectors, a projector is often installed below the central axis of a projection surface onto which images are to be projected from the projector so that the projector does not obstruct the eyesight of viewers. Therefore, a projector is designed such that when the projector is installed such that the central line of a projection lens (projection optical axis) of the projector extends perpendicularly to the projection surface in a horizontal plane, an image, which is projected from the projector onto the projection surface, is positioned upwardly of the projection optical axis, and is displayed in a rectangular shape that is similar to the shape of the display surface of a display device in the projector. The angle that is defined by the plane in which a projector is installed and the line that interconnects the center of the projection lens and the center of the projected image is referred to as “upward angle”. The upward angle is formed by shifting the location where the projection lens is mounted in a vertical direction with respect to the central line of the display device. Projection of an image using a projector which is installed at an upward angle and then tilted vertically or rotated horizontally is referred to as “swing-and-tilt projection”.
FIG. 1 schematically shows an image that is projected at an upward angle alone, an image that is projected at a vertical swing-and-tilt projection angle, and an image that is projected at a horizontal swing-and-tilt projection angle. When an image is projected at an upward angle, rectangular projected image 91 is displayed. When the projector is tilted vertically upwardly for swing-and-tilt projection, rectangular projected image 91 is changed to projected image 92 that has an inverted trapezoidal distortion. However, the inverted trapezoidal distortion in projected image 92 can easily be corrected, because the left side and the right side are equally distorted. When the projector is then rotated horizontally for swing-and-tilt projection, in addition to the vertically upward tilting, and the projection optical axis is tilted at an angle other than the right angle with respect to the projection surface in the horizontal plane, projected image 92 is changed to projected image 93 having a distorted quadrangular shape rather than a simple trapezoidal distortion, because of the upward angle.
In order to correct the distortion of a projected image, highly sophisticated technologies are required for the components of a projector. Various technological efforts have been made to enable easy correction of the distortion. For example, Japanese Patent No. 3519393 discloses a projector having a means for measuring a vertical angle and an adjustment ring to correct the distortion of an image when the image is projected obliquely. Japanese Patent Laid-open Publication No. 2002-044571 discloses a technology for easily correcting the distortion of an image by pointing to four corners of an image using the frame of a projection screen as a reference, when the image is projected obliquely onto the screen.
According to the projector disclosed in Japanese Patent No. 3519393, a trapezoidal distortion can automatically be corrected because a vertical tilt of the projector is acquired by an acceleration sensor. However, when an image is projected from the projector at a horizontal swing-and-tilt angle, an operator has to operate the adjustment ring through a visual process to correct the distortion that is caused by the oblique projection of the image. If the horizontal swing-and-tilt angle is small, then it is possible to make adjustments according to a predetermined procedure. However, if the horizontal swing-and-tilt angle is as large as, for example, about 30 degrees, then it becomes difficult for the operator to properly correct the distortion using the adjustment ring, because the operator, who makes visual adjustments near the projector, is unable to recognize the rectangular image properly when he looks at the projected image from near the projector because of the large horizontal swing-and-tilt angle. Consequently, the operator has to confirm whether the correction is appropriate or not from a position that is right in front of the projection surface each time an adjustment is made, or he has to make adjustments while being informed of the state of the image by another person who is standing right in front of the projection surface.
According to the technology disclosed in Japanese Patent Laid-open Publication No. 2002-044571, if a screen or the like is installed, then the corners of an image that is to be corrected can be specified based on the frame of the screen. Therefore, this technology is free of the problem that an operator is unable to recognize a rectangular shape while viewing the projection surface obliquely, as is mentioned for Japanese Patent No. 3519393. However, if an image is projected onto a wall of a room or the like, then the operator alone is unable to make corrective adjustments because no screen frame is available for use as a reference for such adjustments.